Ported design must go lower than sealed VP100. But only if you make use of the ports. Putting a M2 in that hole probably wouldn't make use of the ports. I never noticed the M2 going lower myself, but this is probably because I never mounted it to the wall as a speaker by itself. I put it on top of something to hold it in place for a few days while I tried it as a center. Then later mounted it above the TV with the VP100 below the TV. It could be that it wasn't close enough to the wall to register the lower notes.

The small on wall bookshelf designs have ports on the bottom of the speaker allowing the bass to escape and reverberate (?) off of the wall below them, perhaps amplifying it in the process. So a good mounting location is probably a requirement for them.

Do you have room there for the on wall 150? I think it would play nicer with the M60s. the M2's would be my next choice as the M3's don't match all that well (I have tried them before with M80's). I recently set up a friend's living room with on wall 22's and a 150, with a sub and was pretty surprised at how well it sounds. I'm actually a fan of the on wall speakers. They do quite well when considering their physical limitations.

Mike, I think that the onwall VP150 is too wide - i.e. it would extend across not only the center opening and the vertical rails, but into the openings on the sides. But I had not seriously considered that option, so I will measure for it. Thank you for the suggestion.

I know you guys want to make the round hole square so that it fits, but my goal is to get "something better than a VP100v2 that works in this configuration" not necessarily just "a better center channel".

I think just trying an M2, then send it back if not satisfied is probably warranted at this point. Without being mounted on a wall, I'd go with an in-wall model, mounted into a piece of wood that fits the hole. Some renovation, but very minor. Can build using hinges on inside to make it easy to tilt upward.